1. Field of the Invention
The present invention relates to a magnetic transducer head having a magnetoresistance effect, and more particularly to a magnetoresistance effect type magnetic head apparatus having bias means.
2. Description of the Prior Art
A magnetoresistance (hereinafter referred to as "MR" effect) effect type magnetic head apparatus has a head member h with a structure as shown in FIG. 1A and FIG. 1B. FIG. 1A is a sectional view of an essential part of an MR head, and FIG. 1B is a plan view thereof. On a magnetic substrate 1 of Ni-Zn ferrite or Mn-Zn ferrite, or through an insulating layer 2 of SiO.sub.2 etc. on the substrate 1 if it is conductive, a bias conductor 3 of band-shaped conductive film is applied. This forms a bias magnetic field generating current passage for applying a bias magnetic field to an MR sensing element as hereinafter described. An MR sensing element 5 comprising MR magnetic thin film on Ni-Fe alloys or Ni-Co alloys is arranged on the bias conductor 3 through an insulating layer 4. A pair of magnetic layers 7 and 8 of Mo permalloy or the like to form a magnetic core of part of a magnetic circuit so that the magnetic layers 7 and 8 ride at each one end on the MR sensing element 5 through a thin insulating layer 6 and extend across the bias conductor 3 and the MR sensing element 5. A protective substrate 10 is provided for the substrate 1 through a non-magnetic protective layer 9. An operating magnetic gap g is formed between one magnetic layer 7 and the front end of the substrate 1 through a non-magnetic gap spacer layer 11 comprising, for example, the insulation layer 6 having a required thickness. At a front surface of the substrate 1, the gap spacer layer 11, the magnetic layer 7, the protective layer 9 and the protective substrate 10 are polished so as to form an opposing surface 12 for a magnetic recording medium. The magnetic gap g is formed there and faces the recording medium. The rear end of the magnetic layer 7 which forms the magnetic gap g and the, front end of the other magnetic layer 8 are formed to rest on the MR sensing element 5 through the insulating layer 6, and both ends are spaced from each other by a discontinuous portion 13. The rear end of the magnetic layer 7 and the front end of the magnetic layer 8 are electrically insulated from the MR sensing element 5 by the insulating layer 6, but are magnetically connected thereto. The discontinuous portion 13 between both magnetic layers 7 and 8 is magnetically connected by the MR sensing element 5, so that a magnetic circuit is formed around the substrate 1, namely: the magnetic gap g--the magnetic layer 7--the MR sensing element 5--the magnetic layer 8--the substrate 1.
FIG. 2 shows an enlarged sectional view of an MR type head apparatus of a so-called shield type as another example. In the head apparatus of FIG. 2, a bias conductor 3 and an MR sensing element 5 opposed thereto are positioned between high permeability magnetic bodies 60 and 61 such as ferrite through a non-magnetic layer 62, and one end surface is polished so as to form a tape opposing surface 12.
In such an MR type magnetic head apparatus, signal magnetic flux from the front gap g opposed to the magnetic recording medium flows in the MR element through the above-mentioned magnetic circuit in the case of the MR head of FIG. 1A, or directly in the MR element in the case of FIG. 2. Thus a resistance value of the MR sensing element 5 varies in response to the external magnetic field by the signal magnetic flux. Variation of the resistance value is detected as a voltage variation across the MR sensing element 5 while sensing current flows through the MR sensing element 5. Thus thereby reproduction of the recording signal on the magnetic recording medium is effected. In this case, the MR sensing element 5 must be magnetically biased in order that the MR sensing element 5 acts linearly as a magnetic sensor and has high sensitivity. The bias magnetic field is applied by the magnetic field generated by energizing the bias conductor 3 and by the magnetic field generated by the detecting current itself flowing through the MR sensing element 5.
In the MR type magnetic head apparatus as clearly seen in a schematic constitution of FIG. 3, the MR sensing element 5 is applied with the generated magnetic field while a prescribed d.c. current i.sub.B flows through the bias conductor 3, and at the same time a prescribed sensing current i.sub.MR flows through the MR sensing element 5. In such a state, the MR sensing element 5 is applied with the bias magnetic field HB composed of the magnetic field generated by energizing the bias conductor 3 and the magnetic field generated by the detecting current flowing through the MR sensing element 5. In such a bias condition the signal magnetic field H.sub.S is applied from the magnetic recording medium. A voltage across the MR sensing element 5, based on the resistance variation by the signal magnetic field H.sub.S, i.e. the variation of potential at point A, is amplified by an amplifier 14 and is detected at an output terminal 15. Numeral 16 designates a coupling condensor.
FIG. 4 shows a working characteristic curve of the MR sensing element 5 illustrating the relation between the magnetic field H and the resistance value R. It is clear from FIG. 4 that the resistance R follows a parabolic curve which is convex in an upward direction in the range of the magnetic field H and is small in absolute value, i.e. -H.sub.BR .about.+H.sub.BR. However, the resistance R deviates from the parabolic curve and gradually approaches the value R.sub.min when magnetization of the MR magnetic thin film at a center portion becomes saturated in the magnetic circuit direction. The maximum value R.sub.max of the resistance R indicates that the magnetization of the MR magnetic thin film is directed entirely in the current direction. The bias magnetic field H.sub.B is applied at the characteristic portion according to a parabolic curve in the working characteristic curve, and the signal magnetic field shown by numeral 17 in FIG. 4 is applied from the magnetic recording medium. Then, corresponding to the signal magnetic field, an output according to the variation of the resistance value as shown by numeral 18 in FIG. 4 is obtained. In this case, however, the greater the signal magnetic field, the greater the second harmonic distortion.
In the MR type magnetic head apparatus, the potential at point A of FIG. 3 is determined by the fixed component and variable component of the resistance in the MR sensing element 5. Since the fixed component in this case is about 98% and is largely dependent on temperature, the temperature drift of the potential at point A becomes large. The resistance value R in the MR sensing element 5 is represented by the following formula: EQU R=R.sub.0 (1+.alpha.cos.sup.2 .theta.) (1)
wherein R.sub.0 stands for the fixed component of resistance, .alpha. stands for the maximum resistance variation factor, .theta. stands for the angle between the current direction and the magnetizing direction in the MR sensing element 5. For example, if the MR sensing element 5 is an MR magnetic thin film of 81Ni-19Fe alloy (permalloy) with thickness 250 .ANG., the measured value of .alpha. becomes about .alpha.=0.017. The value of .alpha. in this case is dependent more or less on the thickness or the material of the MR magnetic thin film of the MR sensing element 5, and becomes about .alpha.=0.05, at the most. On the other hand, R.sub.o is represented by following formula: EQU R.sub.o =R.sub.i (1+a.DELTA.t) (2)
wherein R.sub.i stands for the initial value of resistance, a stands for the temperature coefficient, and .DELTA.t stands for the temperature variable component.
The measured value of the temperature coefficient a in the above example of the MR sensing element 5 is about a =0.0027/deg. This may produce a large noise when detecting the d.c. magnetic field. In order to avoid the temperature dependence in the MR magnetic head apparatus, it is usual in differential construction techniques to cancel the temperature dependence.
Moreover, in such an MR type magnetic head element, since the temperature coefficient is large as above described, for example, when heat generated by energizing the MR sensing element 5 or by the bias current flowing through the bias conductor 3 is radiated in unstable fashion by at rubbing of the heat element with the magnetic recording medium, the head temperature thus varies, and a large noise, i.e. a so-called rubbing noise, may be produced.
If the amplifier 14 in FIG. 3 has a low-impedance input, assuming that the cut-off frequency by the capacitor 16 is f.sub.o, the required capacitance C for the capacitor 16 becomes ##EQU1## wherein .omega..sub.o =2.pi.f.sub.o.
If the MR sensing element 5 is made of the permalloy with a thickness of 250 .ANG. and length of 50 .mu.m, the resistance value R becomes about 120.sup..OMEGA.. If f.sub.o =1 kHz, the value of C must be as large as C=1.3 .mu.F. This becomes a problem particularly when the magnetic head apparatus of a multi-track type is formed.
Permeability in a magnetic circuit, particularly that in the magnetic layers 7 and 8 having a relatively small thickness and sectional area, is preferably as large as possible Since the permeability becomes maximum when the external magnetic field is zero, the application of the above-mentioned bias magnetic field lowers the permeability.
The above-mentioned MR type magnetic head apparatus in the d.c. bias system is advantageous in that the effective track width is large and a narrow track is easily implemented. On the contrary, it is disadvantageous since the linearity is bad, the d.c. reproduction is difficult, the rubbing noise is large, the Barkhausen noise is large, and dispersion of the output is large.
In the prior art, an MR type magnetic head apparatus particularly for removal of second harmonic distortion of the output signal has been proposed. Such a magnetic head apparatus will now be described referring to FIG. 5. A head member h is composed of an MR sensing element 5 with the neutral point grounded and two parts 5a, 5b having equal characteristics, and of a bias conductor 3 with the neutral point grounded and two parts 3a, 3b having equal characteristics. Both ends of the MR sensing element 5 is supplied with the same detecting current i.sub.MR in reverse directions from each other. Both ends of the bias conductor 3 are also supplied with the same d.c. current i.sub.B in reverse directions from each other. Thus, the parts 5a, 5b in the MR sensing elements 5 are applied with the bias magnetic field HB in reverse directions from each other on the basis of the magnetic field generated by the d.c. current i.sub.B flowing through the two parts 3a, 3b of the bias conductor 3, and the magnetic field is generated by the detecting current i.sub.MR flowing through the MR sensing element 5 and also with the same signal magnetic field H.sub.s from the magnetic recording medium. A voltage across the MR sensing element 5 based on the resistance variation by the signal magnetic field H.sub.s, that is, the variation of potential at points A.sub.1, A.sub.2, is supplied to a differential amplifier 14'. In this constitution, the points A.sub.1, A.sub.2 have output voltages in inverted phase from each other but with second harmonics in the same phase. Thus an output signal with little distortion results by removing the second harmonics at the output side of the differential amplifier 14', i.e. at an output terminal 15.
However, the MR type xagnetic head apparatus of FIG. 5 in the prior art has the following disadvantages. Since equalization of characteristics at the two parts 5a, 5b of the MR sensing element 5 with a high accuracy is difficult, and equalization of the magnetic field to the two parts 5a, 5b of the MR sensing element 5 with high accuracy is also difficult, an offset may be produced in the output signal. Since a non-sensitive region is produced at the border between the two parts of the MR sensing element 5, a width of the head element 5 cannot be narrowed appreciably and therefore the multi-channel apparatus cannot be easily implemented. An increase of the number of leads for the element also makes difficult the implementation of the multi-channel apparatus.
An MR type magnetic head apparatus of a barber pole type also has been proposed. In this apparatus, a number of conductor bars of gold or the like in parallel to each other are adhered to the MR sensing element in the MR type magnetic head element in an oblique direction to the longitudinal direction of the MR sensing element.
The MR magnetic apparatus of a barber pole type is advantageous in that dispersion of the output is little and the circuit may be formed by an amplifier only. On the contrary, it is disadvantageous since the d.c. reproduction is difficult, the rubbing noise is large, the narrow track cannot be implemented easily, and the effective track width is not very large.